I don't think Apple should be worried. They are still relying on Samsung, just using TSMC as second source to keep pressure on Samsung for low price. Pretty standard technique almost every big fabless company uses this trick.

@p_g- But can Apple continue to rely on Samsung for producing its processors while at the same time engaging in a legal battle with the South Korean firm over the Galaxy Tab? Seems like something's gotta give.

This may be another good example of how Apple respect its designers. Design team of A6 may need more time to optimize product and specification and they may need one or more run. It is is not good practice to fix product launch date before design team agrees to design progress. One wrong example from Apple was iPhone4 Antenna. They should have given more time to RF/Microwave engineers for testing and refinement.

Any stacked memory dies would have to be compatible with the new package, but we did not hear about adjustments by them.
For Xilinx, FPGA dies were side by side on same interposer, but all the dies were made at tsmc, as well as interposer.
I don't know why Apple and tsmc would take such risk so soon. I think the 28 nm yield is hard enough already, and could be the real issue.